Lithography is used in microelectronics manufacturing, particularly in the manufacture of semiconductors, to transfer an integrated circuit pattern to a photoresist film on a substrate. Radiation, such as light, is spatially modulated through a pattern on a mask or reticle to form an aerial image that exposes the radiation-sensitive photoresist film according to the mask pattern. The photoresist is developed, or the pattern may be transferred to the substrate by an etching step, followed by removal of the photoresist.
Because the exposure and focus of the aerial image relative to the photoresist may vary due to variations in substrate reflectivity, film thickness, or topography, it is necessary to continually monitor the transferred patterns to verify that the dimensions of the patterns are within an acceptable range. The importance of such monitoring increases as the size of the features being produced decreases. The difficulty in monitoring such features also increases, however, as the size of the features decreases. This difficulty is exacerbated for features having a size on the order of one micron or less. This is because the preferred method of using a scanning electron microscope (SEM) for performing inspections tends to be relatively slow in operation and difficult to automate for features of a smaller size. The use of optical tools permits faster and more readily automated operations to be implemented, but such optical techniques are inadequate to resolve features of a smaller size, particularly those having dimensions of less than about one micron.
To overcome this problem, U.S. Pat. No. 5,629,772 (issued to Ausschnitt, assigned to the assignee of the present invention, and incorporated by reference) discloses an optical metrology method used in the manufacture of microelectronics. Essentially, referring now to FIGS. 1 and 2, this method comprises using a lithographic process to create a pattern 28 comprising an array of elements 44 on a substrate 45, each element 44 in the array having a length "L" and width "W," with spaces 46 between adjacent elements 44 also having a width W. Substrate 45 may have other layers on its surface, such as layers 45' and 45" as shown in FIG. 2, which may typically comprise silicon dioxide and silicon nitride, respectively.
Typically, width W of elements 44 and spaces 46 between adjacent elements 44 corresponds to the minimum feature dimension for the lithographic process used to create the elements 44. In contrast, length L is larger than the minimum feature. The known method comprises measuring the larger length L of elements 44 as created by the lithographic process, calculating the change in length L of the elements 44 from the nominal length of the elements 44, and calculating the lithographic process bias of the minimum feature from the change in length of the array elements 44. The term "bias" is used to describe the difference between the dimensions of a feature as actually created by the lithographic process and the corresponding nominal dimensions of the pattern 28 desired to be created.
One potential problem with monitoring minimum features in this way is the interaction of sub-layer films with the estimate of the minimum feature size. As the thickness and optical characteristics of the underlying films change, a bias in the minimum feature width is often caused by interference effects between the light reflected from the various regions within and outside the minimum feature structure. Such a bias can be substantially reduced by eliminating the zero order or specular component in the image formation process, such as by using darkfield imaging methods.
A number of darkfield methods have been proposed that provide zero order rejection in the context of optical alignment of wafer structures and for detection of minimum features using optical imaging and polarization rejection. See, generally, Bobroff et al., "Alignment Errors from Resist Coating Topography," J. Vac. Sci. Tech., Vol. B6(1) (Jan/Feb 1988), and co-pending U.S. patent application Ser. No. 09/159,240 (Progler et al.). Methods for higher-resolution measurement of minimum feature size while accounting for sub-layer film bias, however, as well as edge-detection methods that may be implemented on existing brightfield microscopy equipment with minimum modifications, are still desired.